Abstract:

A quantum critical point (QCP) is currently being conjectured for the BaFe₂(As₁₋ₓ Pₓ )₂ system at the
critical value xc ≈ 0.3. In the proximity of a QCP, all thermodynamic and transport properties are
expected to scale with a single characteristic energy, given by the quantum fluctuations. Such a
universal behavior has not, however, been found in the superconducting upper critical field Hc2. Here
we report Hc2 data for epitaxial thin films extracted from the electrical resistance measured in very
high magnetic fields up to 67 Tesla. Using a multi-band analysis we find that Hc₂ is sensitive to the QCP,
implying a significant charge carrier effective mass enhancement at the doping-induced QCP that is
essentially band-dependent. Our results point to two qualitatively different groups of electrons in
BaFe₂(As₁₋ₓ Pₓ )₂ . The first one (possibly associated to hot spots or whole Fermi sheets) has a strong mass
enhancement at the QCP, and the second one is insensitive to the QCP. The observed duality could also
be present in many other quantum critical systems.